Apache Solr Proximity Search and sort by Distance

I wrote a custom module to perform a search of an index I created using Search API and Search API Solr Search. The search is a proximity search that finds the closest locations using a bounding box, but I can't figure out how to SORT the results I get back by the closest. Here is my module code in entirety:

Basically I exposed my latitude and longitude to Solr and used those to set the bounding box and this works find to return results close to me, but I don't see any field in the results set that would allow me to sort by the node with the closest location.

I also tried installing the Search API Location module, but I still can't find anything in the results set to sort by.